The present invention relates to a high density optical storage system; and, more particularly, to a run-length limited encoding method and apparatus for use in a high density optical data storage system.
Currently, a compact disc (CD) and a digital versatile disc (DVD) are two most popular optical recording media. When transmitting or recording data on such a recording medium, the data are modulated so as to be suited for transmission or recording. Block coding has been known as one of such modulation techniques. A code produced by block coding is termed as a variable length code (d,k;m,n;r), wherein d is a minimum number of same consecutive symbols, representing a minimum run-length of zeroes (0""s); k is a maximum number of same consecutive symbols, indicating a maximum run-length of zeroes (0""s); m represents input bit information; n is a modulated code word; and r represents a maximum constraint length.
When recording a variable length code on, for example, an optical disc, it is modulated into a so-called non-return to zero (NRZ) code and recorded based upon the NRZ modulated variable length code, referred to hereinafter as a recording waveform data string.
With a minimum length Tmin between transitions of the recording waveform data strings and a maximum length Tmax between transitions thereof, it is desirable from the viewpoint of the recording density that Tmin be long, i.e., the minimum run-length d be large while it is desirable from the viewpoint of the clock reproduction and jitter that Tmax be short, i.e., the maximum run-length k be small. A variety of modulation methods have been proposed to satisfy the above conditions. It should be noted that the Tmin, being fixed by the nature of the laser beam, can be referred to as a minimum mark length since the optical recording is dependent on the laser beam and there is a physical limit in the size (or length) of the laser beam spot.
Conventionally, the modulation code for a CD and that for a DVD are a so-called eight-to-fourteen modulation (EFM) code and an EFMPlus code, respectively. Both of the EFM code and the EFMPlus code have a minimum run-length constraint d=2 to reduce intersymbol interference (ISI) and a maximum run-length constraint k=10 to recover a timing signal.
The EFM code encodes 8 bit input data to a 14 bit output code word. There are 256 code words corresponding to 256 input patterns of 1 byte. In addition to this, it is necessary to use 3 merging bits between code words in order to suppress direct current (DC) content of the encoded channel bit sequence. Thus, the overall coding ratio of the EFM code is 8/17.
Meanwhile, EFMPlus code, which is an advanced version of EFM code, encodes 8 bit input data to a 16 bit code word. This code utilizes a supplementary code table to suppress DC content of a channel bit sequence in addition to a main code word mapping table. Thus, a code word is selected from these two tables depending on a running digital sum (RDS) thereof. Therefore, the overall coding ratio of the EFMPlus code, which is 8/16, is larger than that of the EFM code, i.e., 8/17. The density ratio of the EFM code and that of the EFMPlus code are 1.41 and 1.5, respectively, wherein the density ratio is defined as DR=(1+d)R, R being the coding ratio.
Conventionally, in recording run-length limited (RLL) modulated code words with the minimum run-length d=2 by employing the EFM code, code words of 3T are recorded within Tmin (Tmin is fixed), 1T being the recording length of one channel bit. Therefore, the conventional modulation codes have limitations on increasing the data recording density. For example, in a standard for 2.5 Gbyte DVD-RAM, Tmin (=3T) is 0.614 xcexcm; accordingly, the length of one channel bit corresponds to 0.205 xcexcm (=(0.614 xcexcm)/3) and the user data bit becomes 0.409 xcexcm to satisfy the coding ratio 8/16. In other words, for recording the user data of 1 byte (=8 bits), channel bits of 3.28 xcexcm(=16xc3x970.205 xcexcm) are needed since the coding ratio is 8/16.
In view of the foregoing, it is necessary to provide a new RLL encoding method to increase a data recording density thereof by further decreasing the channel bit length in comparison with the conventional RLL encoding method employing an EFM or EFMPlus code.
It is, therefore, a primary object of the present invention to provide a run-length limited (RLL) encoding method and apparatus to increase a data recording density thereof for use in a high density optical storage system.
It is another object of the present invention to provide a run-length limited (RLL) decoding method and apparatus for use in a high density optical storage system.
In accordance with one aspect of the present invention, there is provided a run-length limited (RLL) encoding method used in an optical data storage system for encoding an input data of two bits into a corresponding output data O4O3O2O1O0 of five bits,
wherein a minimum run-length of zeroes (0""s) in a stream of the output data is 3;
a maximum run-length of zeroes (0""s) in the stream of the output data is 11; and
the number of 1 included in binary numbers of five bits expressing the corresponding output data is 1 or 0.
In accordance with another aspect of the present invention, there is provided a run-length limited (RLL) encoding apparatus used in an optical data storage system for encoding a current input data C1C0 of two bits into a corresponding current output data O4O3O2O1O0 of five bits, comprising:
a first shift register for storing updated next input data of two bits inputted thereto in response to a predetermined clock signal CLK and for providing data of two bits previously stored therein as next input data N1N0 of two bits;
a first delay circuit for delaying the next input data N1N0 to thereby generate updated current input data;
a second shift register for storing the updated current input data inputted thereto and for providing data of two bits previously stored therein as current input data C1C0 of two bits;
a second delay circuit for delaying the current input data C1C0 of two bits to thereby generate updated past input data;
a third shift register for storing the updated past input data inputted thereto and for providing data of two bits previously stored therein as past input data P1P0 of two bits;
an encoding circuit for encoding the current input data C1C0 of two bits into corresponding current output data O4O3O2O1O0 of five bits by using the past input data P1P0, the current input data C1C0 and the next input data N1N0, wherein a minimum run-length of zeroes (0""s) in a stream of the output data is 3; a maximum run-length of zeroes (0""s) in the stream of the output data is 11; and the number of 1 included in binary numbers of five bits expressing the output data is 1 or 0;
a divider for dividing the predetermined clock signal CLK by 2 to thereby provide a five bit data output clock signal; and
a data output buffer circuit, in response to the five bit data output clock signal, for generating the corresponding output data O4O3O2O1O0 of five bits.